In order to control an anti-lock brake system (ABS) or traction-control system (TCS), it is necessary to detect the RPM of the wheels. Therefore, a rotation-support apparatus with rotation sensor device for drive-wheel is necessary in order to support the drive wheels such that they rotate freely with respect to the suspension and to detect the RPM of the drive wheels. Of this kind of rolling-bearing unit with rotation sensor device, an apparatus for measuring the RPM of the drive wheels has been known and disclosed as in Japanese Patent Publication No. Tokukai Hei 9-21823.
FIG. 5 shows the rotation-support apparatus with rotation sensor device for drive-wheel as described in this publication. This rotation-support apparatus with rotation sensor device for drive-wheel comprises: a knuckle 1, outer race 2, hub 3, inner race 4, a plurality of rolling elements 5, constant-velocity joint 6, encoder 7, rotation detection sensor 8, and first to third seal rings 9 to 11.
Of these, the knuckle 1 is part of the suspension and has a support hole 12 for supporting the outer race 2.
Also, the outer race 2 has a double row of outer-ring raceways 13a, 13b formed around its inner peripheral surface and has an outward facing flange-shaped installation section 14 formed around its outer peripheral surface. This outer race 2 is securely fastened with bolts (not shown in the figure) to the knuckle 1, such that part of the outer race 2 which is further on the axially inside than the installation section 14 (the axially inside is the direction toward the center in the width direction when installed in the automobile, or in other words the right side in the drawings, and this will be the same throughout this description) is fitted into the inside of the support hole 12.
Moreover, the hub 3 has a flange 15 formed around its outer peripheral surface on the axially outside end (the axially outside is the side toward the outside in the width direction when installed in the automobile, or in other words the left side in the drawings, and this will be the same throughout this specification) for fastening to and supporting the wheel (drive wheel) (not shown in the figure), and also has a first inner-ring raceway 16 formed around its outer peripheral surface in the axially middle section, a small-diameter stepped section 17 formed on the axially inside end, and a spline hole 18 formed through its center.
The inner race 4 fits around and is fastened to the small-diameter stepped section 17. This inner race 4 has a second inner-ring raceway 19 formed around its outer peripheral surface, and is fitted onto the small-diameter stepped section 17, and the axially outside end surface of the inner race 4 comes in contact with a step surface 20 on the axially outside end of the small-diameter stepped section 17 such that the axially inside end surface protrudes further inward in the axial direction than the axially inside end surface of the hub 3.
A plurality of rolling elements 5 are located between each of the outer-ring raceways 13a, 13b and first and second inner-ring raceways 16, 19, and they support the hub 3 and inner race 4 such that they rotate freely on the radially inside of the outer race 2.
Also, the constant-velocity joint 6 comprises a spline shaft 21 located on the axially outer half, and an outer ring 22 for the constant-velocity joint on the axially inner half. This kind of constant-velocity joint 6 is fastened to the hub 3 by a spline joint between the spline shaft 21 and the spline hole 18, and by tightening a nut 23 around the tip end (axially outer end) portion of the spline shaft 21 that protrudes from the spline hole 18 such that torque can be transmitted to the hub 3. Also, in this state, the axially inside end surface of the inner race 4 comes in contact with the axially outside end surface of the outer ring 22 of the constant-velocity joint, so that this prevents the inner race 4 from separating from the small-diameter section 17 and applies a pre-load to the rolling elements 5.
The encoder 7 is formed out of magnetic metal plate into a circular ring shape having an L-shaped cross section, and its characteristics on the axially inside surface change alternately at equal intervals around in the circumferential direction, and it is supported and fastened on the axially inside end of the inner race 4.
The rotation detection sensor 8 is fastened to the outer race 2 with a circular cover 25 such that the detection portion of the rotation detection sensor 8 is close to and faces the axially inside end surface of the encoder 7.
Furthermore, of the first to third seal rings 9 to 11, the first seal ring 9 comprises a metal core and seal lips, and when the metal core is fitted into and supported by the axially outer end of the outer race 2, the seal lips come in sliding contact around the outer peripheral surface in the axially middle section of the hub 3. Moreover, the second seal ring 10 as well comprises a metal core and seal lips, and when the metal core is fitted into and supported by the axially inside end of the outer race 2, the seal lips come in sliding contact on the outer peripheral surface of the base end of the encoder 7 that is fitted around the axially inside end of the inner race 4. Furthermore, the third seal ring 11 is made entirely of an elastic material, and in the state where the base end of the third seal ring 11 is fastened around the inner peripheral edge of the cover 25, the tip end of the third seal ring 11 comes in sliding contact with the outer peripheral surface of a bell-shaped outer diameter portion 46 of the outer ring 22 for the constant-velocity joint.
In the case of the prior art rotation-support apparatus with rotation sensor device for drive-wheel constructed as described above, the drive wheel that is fastened to the hub 3 is supported such that it rotates freely with respect to the knuckle 1 to which the outer race 2 is fastened, and so it is possible to drive and rotate the drive wheel by way of the constant-velocity joint 6. Also, the rotation-detection sensor 8 faces the axially inside surface of the encoder 7 that is fixed to the hub 3 by way of the inner race 4, and when the hub 3 rotates together with the drive wheel, the output of the rotation-detection sensor 8 changes. The frequency at which the output of this rotation-detection sensor 8 changes is proportional to the RPM of the drive wheel. Therefore, by inputting the signal that is output from the rotation-detection sensor 8 to a controller (not shown), it is possible to find the RPM of the drive wheel and to properly perform ABS or TCS control.
Also, since the first and second seal rings 9, 10 seal the bearing space 26 inside the bearing where the rolling elements 5 are located from the outside, it is possible to prevent foreign matter on the outside from getting inside the bearing space 26 and it is also possible to prevent grease inside the bearing space 26 from leaking to the outside. Furthermore, since the second seal ring 10 seals the bearing space 26 from the detection space 27, in which the encoder 7 and rotation-detection sensor 8 are stored, and the third seal ring 11 seals this detection space 27 from the outside, no foreign matter or grease gets inside the detection space 27.
In the case of the prior art construction described above, the first to third seal rings 9 to 11, which form contact-type seals, seal the bearing space 26 and detection space 27 from the outside, and also seal the both spaces 26, 27 from each other. The surface pressure at the area of contact between the edges of the elastic seal lips of the first to third seal rings 9 to 11 and the opposing surfaces is low, however since the length of the contact area is long, the friction resistance at the area of contact cannot be ignored. Therefore, the increase of the rotation resistance of the rotation-support apparatus with rotation sensor device for drive-wheel due to the first to third seal rings 9 to 11 becomes large enough that it cannot be ignored. This increase in rotation resistance becomes the cause of lowered performance of the automobile, mainly the acceleration performance and fuel-consumption performance, and is not desirable.